In recent years, along with the arrival of a ubiquitous society, the achievement of high-speed performance, multi-functions, and integrated functions of a micro processing unit (MPU) and the achievement of high-speed performance of recording devices such as memory have been taking place in information processing devices, communication devices, or the like, especially in personal computers, mobile phones, and game equipment.
However, noise radiated from these devices or noise conducting through the conductors inside these devices is becoming a problem since it can cause a malfunction of the devices themselves, other electronic equipment, or parts thereof, and can also adversely affect the human body. This noise includes noise caused by an impedance mismatch of conductors provided in a printed wiring board equipped with MPUs, electronic parts, and the like, a noise caused by crosstalk between conductors, and a noise induced by the resonance between a power supply layer and a ground layer which is caused by simultaneous switching of semiconductor elements of the MPU or the like.
As printed wiring boards in which occurrence of such noises are suppressed, the following printed wiring boards are known:
(1) a printed wiring board in which a metallic film made of metal with a resistivity greater than that of copper foil is formed on both surfaces of a power supply layer and a ground layer, the power supply layer and ground layer being formed by a copper foil (Patent Document 1);
(2) a printed wiring board in which a film containing a conductive substance, the film having an anisotropic conductivity in the vertical direction with respect to the printed wiring board surface, is formed on both surfaces of a power supply layer and a ground layer, the power supply layer and ground layer being formed by copper foils (Patent Document 2); and
(3) a printed wiring board having a parallel flat plate structure composed of a power supply layer and a ground layer, in which the power supply layer or the ground layer is formed by integrating a resistive conductor film and a pattern for supplying electric current to electronic parts, and thickness of the resistive conductor film is no more than 1/10 of that of the pattern for supplying electric current to electronic parts (Patent Document 3).
In the printed wiring board (1), it is considered that a high-frequency eddy current flowing through the surface of the copper foil can be attenuated therein, as a result of which an electric potential of power supply or the like can be stabilized and the radiation of unwanted noises can be suppressed even if semiconductor elements are simultaneously switched. In order to attenuate the high-frequency current (surface current) flowing through the surface of a conductor using a metal film with a thickness of several micro meters, the thickness being approximately equal to the surface depth, a material with a considerably high resistivity is required depending on the frequency of the high-frequency current to be attenuated. However, it is practically impossible to acquire such a material, and as a result, the printed wiring board (1) cannot exhibit sufficient noise suppressing effects.
Also, in the printed wiring board (2), it is considered that the high-frequency eddy current can be attenuated therein in a similar way to the above. However, a step of forming an anisotropically conductive film with a surface roughness of copper foil equal to or greater than the surface depth is complicated. In addition, the printed wiring board (2) also cannot exhibit sufficient noise suppressing effects.
Also, in the printed wiring board (3), it is considered that electromagnetic interference (EMI) can be suppressed therein. However, a step of integrating a resistive conductor film and a pattern for supplying electric current to electronic parts is complicated, since the pattern for supplying electric current to electronic parts needs to be formed on the resistive conductor film by plating, and thus a longer time is required for producing the printed wiring board.
[Patent Document 1] Japanese Unexamined Patent Application, First Publication No. Hei 11-97810
[Patent Document 2] Japanese Unexamined Patent Application, First Publication No. 2006-66810
[Patent Document 3] Japanese Unexamined Patent Application, First Publication No. 2006-49496